The present invention relates to polyurethane compositions, and more particularly, to polyurethane foams, polyisocyanurates, elastomers, coatings and adhesives exhibiting reduced toxic smoke emission on burning. The invention also relates to a method of providing polyurethane compositions having these desirable properties.
Polyurethane compositions are produced in a variety of forms including rigid and flexible foams, polyisocyanurates, elastomers, coatings and adhesives. It is well known that polyurethane compositions can be prepared by reacting an organic polyfunctional isocyanate with an organic compound having two or more reactive hydrogen atoms such as are found on polyesters, polylkylene polyols, polyester amides, polyalkylene ethers, polyacetals and polyalkylene thioethers. When the reaction is conducted under anhydrous conditions, the resulting polyurethane may be non-porus. Urethane elastomers generally are prepared from a diisocyanate, a linear polyester or polyether resin, and a low molecular weight curing agent such as a glycol or a diamine. If a cellular or foamed polyurethane is desired, gas-generating additives such as water or other blowing agents are included in the reaction mixture. Generally, flexible urethane foams are prepared from polyether or polyester resins, an excess of diisocyanate and water. The water reacts with the excess isocyanate groups not previously reacted with the polyether or polyester, and carbon dioxide is formed which is entrapped in the reaction mixture. Auxiliary blowing agents such as volatile hydrocarbons and fluorocarbons also may be employed, and generally are preferred when rigid foams are produced.
It is well known that various materials may be employed as catalysts or activators in the formation of polyurethanes. The most common of the catalysts are the amines, typically tertiary amines such as triethylene diamine. Although the amines are the most commonly used catalysts, they generally are considered to be undesirable because they exhibit high volatility which results in a high rate of loss from the reaction mixture, obnoxious odors and toxicity. Despite these problems, the amines continue to be commonly employed because there are few satisfactory substitutes.
A wide variety of metal catalysts has been suggested for preparing flexible polyurethane foams. See, for example, Polyurethanes: Chemistry and Technology Part II, Saunders and Frisch, Interscience Publishers 1964, pages 21-25 where the catalytic activity of various sodium, lead, iron, tin, copper, manganese, cadmium, cobalt, etc. compounds, particularly salts are discussed. Studies of the activity of various catalysts for rigid foam systems are discussed in the same book at pages 217-219. Tertiary amines and tin compounds such as stannous octoate and dibutyl tin dilaurate are reported to be useful. Additives useful as flame retardants in rigid urethane foams are described on page 223 of the same book and these additives include metal soaps such as zinc stearate, calcium oleate and zinc laurate.
U.S. Pat. No. 3,884,849 describes low density, flexible and semi-flexible urethane foams which contain, in addition to the usual reactants, a small amount of a solid chlorine-containing polymeric resin, zinc oxide and a zinc salt of an organic monocarboxylic acid. The zinc salt is added to the formulation prior to foaming and is added in addition to the normal catalyst. The zinc salt reduces or prevents discoloration of the urethane foam. It is also suggested to use a catalyst combination of a tertiary amine and a metal containing co-catalyst. Non-yellowing polyurethanes also are described in U.S. Pat. No. 3,580,873 wherein isocyanate-terminated prepolymers are reacted with a mixture of monoethanolamine and triethanolamine in the presence of a catalytic amount of a metal drier such as calcium-, zinc-, or lead naphthenate or octoate. U.S. Pat. 3,746,692 describes the preparation of polyurethane compositions by reaction of a polyether polyol with an organic polyisocyanate, a liquid halogen-containing aliphatic modifier compound and one or more organometallic catalysts for urethane formation. Cobalt naphthenate is listed as an example of a catalyst. U.S. Pat. No. 3,808,162 describes the preparation of urethanes using a mixture of a soft acid metal compound such as tin or cobalt octoate with a carboxylic acid having a pK.sub.a greater than 0.23, for example 2-ethylhexanoic acid. U.S. Pat. No. 4,125,487 relates to the formation of polyurethane-vinyl polymers which utilizes a free radical catalyst for the vinyl polymerization and a cobalt salt for the urethane formation.
British Pat. No. 980,139 describes a process for manufacturing foamed polyurethane materials from a mixture of an organic polyisocyanate, a polymer containing secondary hydroxyl end groups in the presence of catalysts normally used for preparing foams and a zinc salt of a carboxylic acid. It is reported that the use of the zinc carboxylates in conjunction with known catalysts results in decreased reaction time, decreased surface drying time and products having improved physical properties. U.S. Pat. No. 3,347,804 describes a catalyst for preparing urethane foams which comprises a combination of tin, lead and zinc salts.
The above-described prior art represents a sampling of the suggestions which have been made for improving the properties of urethane foams. In spite of the numerous suggestions made in the prior art, research has continued towards developing polyurethane compositions having increased flame retardancy, and more recently, increased efforts have been directed towards developing polyurethane foams which exhibit a reduction in the emission of toxic fumes when burned.